The invention relates to variable speed belt driven transmission systems, but more particularly, the invention relates to speed sensing pulleys and methods for improving axial forces at the driver pulley of the transmission for improved shifting characteristics.
Adjustable speed V-belt drives are variable speed belt transmission systems which are either manually or automatically regulated. Belt driven transmissions are used in various machinery such as agricultural equipment, snowmobiles, automobiles and industrial equipment. The drives are powered at some peak load by some source such as a motor, and may be required to deliver power at various speed ratios and torques to a constantly changing output load. In automotive applications, for example, an internal combustion engine having peak and transient torque charcteristics, delivers power at various speed ratios through a transmission to vehicle wheels that react to changing road loads (e.g., windage, hills, and speed). Belt driven transmissions customarily are designed to automaticaly shift to accommodate changing road loads.
The prior art is replete with examples of automatic pulley shifters or actuators that are either speed responsive, torque responsive, or combinations thereof. The shifters may be mechanically operated, electrically operated, pneumatically or hydraulically operated. A speed responsive system may use centrifugal fly weights, and a torque responsive system may use an actuator with a helical torque ramp or a hydraulic pressure that is generally related to torque. This invention is primarily directed to a predominantly mechanically controlled belt driven transmission system which uses a torque sensing driven pulley and a speed sensing driver pulley.
In some transmission applications, it is desirable that a variable speed belt transmission approach shifting at a constant input speed and thereby transmit constant power. To do this, axial forces at the speed sensing driver pulley must be balanced through a variable speed belt to axial forces at a predominantly torque sensing driven pulley. The degree of variance of a transmission from shifting at a constant speed depends on how well the driver pulley axial forces are balanced against the driven pulley axial forces through an interconnecting variable speed belt. The degree of driver pulley, driven pulley axial force mismatch is reflected in variations of shifting speed. Predominantly mechanically operated transmissions have variations in shifting speed as high as 20 percent of a desired shifting speed. The difficulty with prior art variable speed drives of not being able to closely approach shifting at a constant speed is caused by the axial forces of a predominantly mechanically operated driver pulley not being matched in a force balance to the axial forces of a torque sensing driven pulley.
There are several references that analyze axial forces of variable speed V-belt drives. B. G. Gerbert has published several papers on variable speed drives and some of his works are: (1) "Force and Slip Behavior and V-belt Drives." "Acta Polytechnica Scandinavica, MECH."-A&G., Series No. 67, Helsinski, 1972; (2) "Adjustable Speed V-belt Drives-Mechanical Properties and Design." SAE Paper 740747 (1974); (3) "Doctors Thesis on V-belt Drives With Special Reference to Force Conditions, Slip, and Power Loss." Lund Technical University, Lund, Sweden, (1973); and (4) "A Complimentary Large Slip Solution in V-belt Mechanics." ASME Paper 77-DET-162 (1977).
Reference (1) Supra, analyzes various types of adjustable speed V-belt drives and at page 5, example 5, a driven pulley with a torque ramp for closing the pulley halves together responsive to rotational changes is discussed in conjunction with FIG. 3 showing driven pulley axial forces as a function of the coefficient of traction. The axial force/coefficient of traction chart is useful for showing axial force, tension interrelationship for a variable speed belt drive. Dimensionless axial-force, F/(T.sub.1 +T.sub.2) where F is axial force, T.sub.1 is tight side load tension and T.sub.2 is slack side belt tension, is scaled on the ordinate and traction ratio (T.sub.1 -T.sub.2)/(T.sub.1 +T.sub.2), is scaled on the abcissa. Such charts show that dimensionless axial force at the driven pulley is generally within a constant band for all traction ratios and speed ratios. Comparatively, dimensionless axial force at the driver pulley drastically increases with traction ratio for all speed ratios. Thus, the axial forces at the driven pulley generally define the total tension (T.sub.1 +T.sub.2) in the drive as well as the force available to produce torque (T.sub.1 -T.sub.2) for transmitting power. The charts shows the interrelationship for matching driver pulley to driven pulley axial forces for any given traction ratio and where the driver pulley axial forces are generally greater than the driven pulley axial forces except at low traction ratios. Of course, the interrelationship between axial force and a traction ratio is influenced by belt design, pulley diameter, and pulley center distance. These interrelationships are also discussed in the above references.